Numerous implantable medical devices are available for acute or chronic implantation within patients. Some implantable medical devices may be used to chronically monitor physiological signals of the patient, such as implantable hemodynamic monitors, implantable cardiac monitors (sometimes referred to as implantable loop recorders or ECG monitors), implantable blood chemistry monitors, implantable pressure monitors, or the like. Other implantable devices may be configured to deliver a therapy in conjunction with or separate from the monitoring of physiological signals.
For example, in some medical applications, it may be advantageous to acquire both biopotential signals, such as intracardiac electrogram (EGM) signals, electrocardiography (ECG) signals, electromyogram (EMG) signals, or electroencephalogram (EEG) signals, to monitor electrical activity of a tissue or organ as well as bioimpedance signals, which may be correlated to a volume, pressure, fluid status or other characteristic of the monitored tissue or organ. In order to measure bioimpedance, an excitation or drive signal is applied to an electrode pair such that the resulting impedance between the electrodes can be determined, e.g. based on a voltage signal measured across the electrode pair after applying a drive current signal. The drive signal, if applied to the same pair of electrodes used for acquiring a biopotential signal, however, can produce significant noise and artifact on the biopotential signal. Typically, therefore, a biopotential signal and a bioimpedance signal are monitored using two different electrode pairs to avoid interference between the drive signal and the recorded biopotential signal. The requirement of two separate electrode pairs and corresponding feedthroughs through the device housing, however, increases the required size and volume of an implantable device and is contrary to the goal of reducing device size.
Advances in medical device technology have enabled implantable devices to be made smaller in size, which facilitates minimally invasive procedures for implanting the device and promotes patient comfort. Reduction of device size poses limitations on the space available for batteries, sensors, processing and control circuitry and other device components that support the primary device function. It is desirable, therefore, in a device that is used for monitoring both a bioimpedance signal and a biopotential signal, to eliminate the requirement of two electrode pairs for separately monitoring bioimpedance and biopotential signals.